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• An unsteady mathematical model was developed based on wave theory. • The model was validated by the tested results. • Effects of key parameters on the thermal performance were studied. Seawater heat pump system exhibits merits of reducing building energy consumption, while enhancing the heat transfer of seawater heat exchangers is the key to improve the system performance. The effect of seawater flow behavior on the thermal performance of seawater heat exchanger immersed in oscillating flow is significant but often neglected. An unsteady mathematical model was firstly developed to describe heat transfer of oscillating flow around the staggered tube-bundle heat exchanger and verified by experimental results. Effects of the structural and operational parameters, such as the tube length, heat transfer fluid velocity, wave parameters and sinking depth on the thermal performance were analyzed. It was shown that the thermal efficiency of seawater heat exchanger ascended with the increase of wave amplitude and decrease of the heat exchanger sinking depth. Additionally, the tube length and heat transfer fluid velocity could be reasonably designed to achieve the optimum value between cost and performance. The heat transfer mechanism in seawater side was predominated by forced convection to natural convection with the increasing sinking depth under various wave parameters. The proposed model is favorable to delicately characterize the heat transfer of seawater heat exchanger with the consideration of seawater flow. The results in this paper is helpful to guide the optimization on designing the staggered tube-bundle heat exchanger in oscillating flow and to promote the application of ocean thermal energy. [ABSTRACT FROM AUTHOR]

• The effects of the oscillating flow on the thermal performance were studied. • An empirical correlation with acceptable accuracy was proposed. • Comparisons of various correlations and experimental results were conducted. Owing to the growing demands to develop sustainable energy, ocean energy is attracting increasing attention for the large amount of renewable, available, and potential energy resources stored in the ocean. An experimental system was constructed to study the heat transfer characteristics of staggered tube bundle heat exchanger immersed in oscillating flow. The results revealed that the instantaneous horizontal and vertical velocities of the oscillating flow exhibited a periodic fluctuation trend. Moreover, the cycle-averaged velocity and the external heat transfer coefficients were clearly affected by the wave height, whereas the wave period had little effect. To quantitatively describe the heat convection of staggered tube bundle heat exchanger in oscillating flow, a correlation of the Nusselt number that accounted for the combined effects of the wave parameters (including the wave height and wave period), water depth, and location depth of the submersed tube bundles was proposed. Comparisons with various correlations revealed that previous correlations could not describe the external heat convection in oscillating flow accurately. The newly proposed correlation was proven to predict the external Nusselt number with an acceptable accuracy under various testing conditions, which benefitted the design of the seawater heat exchanger. [ABSTRACT FROM AUTHOR]